This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A216099 #22 Aug 07 2025 20:20:24
%S A216099 4,20,100,500,5000,50000,500000,5000000,50000000,500000000,5000000000,
%T A216099 50000000000,500000000000,5000000000000,50000000000000,
%U A216099 500000000000000,5000000000000000,50000000000000000,500000000000000000
%N A216099 Period of powers of 3 mod 10^n.
%H A216099 <a href="/index/Rec#order_01">Index entries for linear recurrences with constant coefficients</a>, signature (10).
%F A216099 a(n) = 4*5^(n-1) for n <= 4.
%F A216099 a(n) = 5*10^(n-2) for n >= 5.
%t A216099 Table[If[n<5,(4*5^n)/5,10^n/20],{n,20}] (* or *) Join[{4,20,100},NestList[ 10#&,500,20]] (* _Harvey P. Dale_, May 31 2017 *)
%o A216099 (PARI) a(n)=if(n<5,4*5^n/5,10^n/20) \\ _Charles R Greathouse IV_, Mar 26 2016
%Y A216099 Cf. A000244, A001148, A093143, A001218.
%K A216099 nonn,base,easy
%O A216099 1,1
%A A216099 _V. Raman_, Sep 01 2012